There already exist flat wires such as polyimide coated wires; enamel-coated wires; highly heat-resistant, cementing enamel-coated wires, etc. In recent years, there has been great development in the miniaturization of electrical devices and high density assembly technology. This phenomenon has created a demand for the parts used in electrical devices to be heat-resistant. However, the maximum operating temperatures for polyimide coated flat wire; enamel-coated wire; or highly heat-resistant, cementing enamel-coated wire are 250.degree. C., 150.degree. C. to 220.degree. C., and 220.degree. C., respectively. This level of heat resistance is still not satisfactory for the above-mentioned requirements.
In a known flat wire manufacturing process, a cross-sectionally circular conductor is first rolled to give a flat conductor having an approximately rectangular cross-section. The flat conductor is then covered with a varnish solution and baked to give the flat coated conductor. A varnish solution of a benzimidazol-based polymer may be used in this known method to obtain a flat conductor coated with a reaction product of polymers of polybenzimidazol. However, applying a benzimidazol-based polymer varnish to a flat conductor does not always make it possible to obtain a uniform and homogeneous coating. In particular, the angle or corner portions of the flat conductor tend to have a thinner coating or to form pin holes, which makes the wire incapable of resisting high voltages. On the other hand, a process of first plating a cross-sectionally circular conductor and of rolling it thereafter to obtain a flat conductor is described in U.S. Pat. No. 5,483,021. However, this process relates to the plating of a corrosion-resistant material; the mechanical problem with which the Invention is concerned is not approached in this art.